An irrigation controller sometimes uses signals from remote environmental sensors that detect rain events, temperature, solar radiation, wind speed, humidity, soil moisture, evapotranspiration, flow, or other parameters. Typically the irrigation controller and the sensor communicate through a hard wired connection. Many times the hard wired connection uses the same pair of wires to supply power to the sensor, as it does to communicate information back to the irrigation controller. Other times separate wires are used for power and control. One of the reasons the communications link is typically a hard wired connection is the fact that many times the irrigation controller is installed in a vandal-resistant metal cabinet that shields RF signals, making it impractical to use a wireless communications link. Another reason that the communications link is typically a hard wired connection is that an RF receiver circuit typically costs significantly more than a simple hard wire receiver circuit.
As previously mentioned, irrigation controllers conventionally retrieve data from a remote sensor using a two conductor hard wired connection. Typically a pair of wires carries both an excitation voltage to power the sensor as well as data in the form of a modulated current draw. Data containing environmental information of interest is communicated to the irrigation controller by modulating the amount of current drawn by the environmental sensor that is sending the data. Current modulation is typically more noise tolerance and affected less by long wire runs, than a voltage modulated signal.